Power supplies are used virtually in every electronic device primarily for converting alternate current (AC) electrical input that may be vary in voltage due to power line conditions and load factors into a constant direct current (DC) output to power the internal circuitry of the electronic device.
There are several types of power supplies such as a linear regulator, a switching regulator and low drop-out regulator (LDO) are used in different applications. A linear regulator is a system used to maintain a steady voltage. The resistance of the regulator varies in accordance with the load resulting in a constant output voltage. The regulating device is made to act like a variable resistor, continuously adjusting a voltage divider network to maintain a constant output voltage and continually dissipating the difference between the input and regulated voltages as waste heat. A switching regulator uses an active device that switches on and off to maintain an average value of output. A low-dropout or LDO regulator is a DC linear voltage regulator that can regulate the output voltage even when the supply voltage is very close to the output voltage.
For certain applications, the voltage regulator is forced into the so-called pass-through mode (PTM) when the input supply voltage is too low for regulation, in which case the output pass device will be fully opened, clamping the output to the input. Large silicon area is often required to fabricate the output pass device to minimize the resistance and reduce the voltage drop across it. Failing to fully open that device will lead to significant silicon area.